Contents - Previous - Next

Plant genetic resources of the new world
Introduction of flora from the old world and causes of crop marginalization

Plant genetic resources of the new world

Genetic resources
The major phytogeographical regions
Biodiversity of american flora
Cultural relationships
Distribution of genetic diversity
Changes since 1500
The challenge of conservation
Ornamental plants
Medicinal plants

The discovery of America brought about the biggest exchange of germplasm in history. Previously, only three species - one cultivated (sweet potato) and two spontaneous (bottle gourd and coconut) - were common to the agricultures of both the Old and the New World. After 1492, the exchange of cultivated species not only radically changed the diet of the majority of humankind but also led to commercial crops being developed in the tropics and a new world economic order being created. The relationship between human beings and cultivated plants affected everything from basic food and clothing requirements to the use of plants for ornamental purposes and leisure.

The first introductions into America European grains. vegetables and fruits planted in Hispaniola were unsuccessful but, a few decades later on the highlands of Mexico and the Andes. they produced yields which exceeded those of Europe. On the other hand, bananas, sugar cane and citrus fruits, among others, acclimatized easily to tropical areas. From Africa, yams and other minor crops were introduced. From America and Europe, maize, which arrived shortly after the discovery, had spread throughout the centre of the continent with in three or tour decades. Later, potatoes, tomatoes, gourds, beans and chilies were slowly incorporated into European cooking, which thereby assumed its present-day characteristics. In Africa, the cassava or manioc and the peanut were introduced, which radically changed the diet in the western part of the continent. Rice, originating in Southeast Asia and taken to America from Spain and Africa, came to constitute a staple food; on the other hand, cottons from the New World displaced African and Asian cottons. In commercial crops, the influence of exchange was decisive: sugar cane was the first agro-industrial product of the New World while coffee from Ethiopia, introduced two centuries ago, became the main, product of Latin America, followed by the banana, originating from Southeast Asia. In the opposite direction, cocoa from Brazil has become the main commercial product of West Africa and, together with rubber from the Amazon region, an important export item of Southeast Asia. In Latin America during this century, African grasses have replaced the local species, which are few and of low yield.

The exchange of germplasm is a continuous process in agricultural diversification and genetic improvement. The tact that on both sides of the Atlantic most commercial crops are of foreign origin means that their preservation, evaluation and trade is of world interest. Germplasm losses in Southeast Asia may have more effect on tropical America than on that region itself.

Genetic resources

At present, the term "genetic resources" is being employed increasingly instead of germplasm, as it is considered to be broader and more appropriate. Interest in the preservation, documentation and intensive use of genetic resources is relatively recent. As one of the earth's many natural resources, once it is lost the germplasm of cultivated plants may never be recovered. Its concentration in countries with an incipient agriculture creates particular problems regarding its conservation and trade, although the agricultural systems prevalent in those regions tend to preserve it as seed material.

The range of the genetic resources of cultivated plants varies considerably according to the species. The first category consists of the primitive cultivars, i.e. those resulting from an initial selection made by the farmers. There is no complete list, not even for species of high economic value, by which the number of cultivars could be estimated; in some species there may be dozens, in others hundreds. Most have a restricted geographical distribution and many are grown only for family consumption.

Outside their centre of origin they produce high yields. as certain tests reveal. Their potential value consists of the set of genes resistant to very varied environmental conditions, diseases and pests and, often, their high nutritional value content. They are very prone to genetic erosion, as their populations are small and consequently easy to replace with other cultivars and, since they are used by the poorest social groups, they receive scant attention.

The second category, advanced cultivars, is limited in tropical America to a few species; they are the result of genetic improvement efforts intended to produce high-yielding cultivars that are resistant to diseases and pests or adapted to specific environmental conditions. Some crops originating from the American tropics have undergone genetic improvement in regions with advanced agricultural sectors: North America, Europe and Japan. The cultivars that they gave rise to did not, however, adapt to the prevailing conditions in tropical America.

The third category is made up of the cultivated species' wild populations which subsist in the area where the species has been domesticated. They grow spontaneously and sometimes the action of disruptive selection does not allow an exchange of genes between the wild and cultivated populations. Weeds, which are difficult to define, form a separate group. In primitive agricultural systems, there is no clear line between cultivars, weeds and wild populations because the latter two are also utilized in some cases.

In addition to the species, the cultivated or wild relatives with which an exchange of genes is possible constitute a fourth category which may have an important role in genetic improvement. Frequently, the relations of a cultivated species are used in practices, such as grafting, that require a physiological affinity. In some cases citrus fruits and ornamental orchids - the genetic resources are extended to other genera in the formation of multigeneric hybrids.

The major phytogeographical regions

The background to the processes determining the richness of the continent's genetic resources may be understood better if we consider their relationship with the distribution of the plant formations and their biodiversity, and with human factors or, in other words, cultural factors. The former is very complex in the tropical region of the New World because of the number of natural landscapes and the differences between them.

Around lat. 25°N, the arid regions of North America, including deserts and semi-deserts, come to and end and areas of greater humidity begin along the coastal areas of Mexico. Running parallel to the coast are the Sierra Madre mountain ranges, traversed by the high and wide central valleys which formed the heart of Mesoamerica. Towards the south, the mountainous areas, ranges and dividing valleys extend as far as Panama, with a break only in Tehuantepec. As far as Nicaragua, the vegetation of the ranges is predominantly of northern origin, and the first high. bleak Andean plateaus appear in Costa Rica. In the low areas there are notable differences between the two slopes. On the Atlantic slope, the rain forest begins south of Tamaulipas and continues over the coastal plains as tar as Darien in Panama, broken only in Yucatan by a low, dry forest. and in the north of Nicaragua by extensive pinewoods resulting from specific soil conditions. On the Pacific slope, on the other hand, in areas with alternating seasons, the dry tropical forest extends from Sinaloa to central Panama, with a single break through the humid tropical forest of Osa in Costa Rica. The lowland forests are mainly of southern origin and penetrate Mexico as tar as the northern limit of the rain forests or dry forests.

FIGURE 1 Major types of vegetation in Mesoamerica, the Caribbean and South America

In South America, the tropical rain forest of the Gulf of Darien extends south through the coastal plains as far as Ecuador; it branches north and northeastwards in Colombia and Venezuela, where it borders on a dry tropical forest, similar to that of Mexico in Central America, which changes in coastal areas to semi-desert scrubland, with Cactaceae extending as far as the Caribbean coast.

The Andean ranges present a great diversity of vegetation cover, from the slopes bordering the Pacific, which are desert-like in Peru and Chile, to the vegetation of the bleak plateaus. The eastern slope is generally much more humid and forms a continuous strip from Venezuela to Bolivia (cejas and yungas). The high elevation causes areas of extreme dryness in the Andes, some of which are completely devoid of vegetation, for instance the salt marshes.

In the extreme northeast of the Andes, an area of low humidity, the Orinoco plains, extends from the centre of the continent to the Atlantic, ending in very dry forests on the coast. On the more humid ground to the east of the Orinoco River, there are dry forests and palm groves.

The centre of the continent is covered with the humid tropical forest of the Amazon Orinoco, except for the high savannah of Guyana. This forest, the most extensive in the world, can be divided into several subregions according to the types of vegetation and composition of flora, caused by the expansion of the old plant formations and by environmental conditions. What is important from a phytogeographical point of view is the Obidos breech which crosses the Amazon basin from north to south. It is a separate strip with a lower precipitation of around 1 500 mm, much less than the rest of the Amazon region receives, which divides the region into a western and eastern zone and acts as a barrier to the spread of many species. The upper Amazon, from Obidos to the Andean ranges, is one of the most interesting areas in South America on account of its wealth of flora and genetic resources - numerous crops originated here.

The decrease in humidity south of the Amazon forest gives rise to landscapes of open forest and savannah the cerrado is an enormous area in central Brazil which is relatively low, undulating or with mountain ranges which are not very high and are covered with scattered, low evergreen arboreal vegetation which is denser around water currents. The humidity is favourable to agriculture, which has recently been developed and, if chemical fertilizers are used, soils are productive.

Northeastern Brazil is for the most part covered by the caatinga, with a clear, deciduous forest predominated by palms and cacti. The area offers natural products for collection, such as carnauba wax in the extreme north, but it is a difficult area for agriculture. On the rain forest border in areas with about 1 500 mm of precipitation, there are great formations of babassu, which is an important source of oil. The cerrado and the caatinga are poor in native crops, partly because of their abundance of other natural resources and partly because of the adverse conditions for permanent agriculture.

Between the caatinga and the ocean there is a belt of high humidity with rain forest which, in Bahía, is used to a great extent for cacao, cane and other crops. The coastal rain forests are broken to the north of Rio de Janeiro by dry areas. but they continue south almost to lat. 30°.

From the southern limit of the cerrado to the Tropic of Capricorn there are several regions with different landscapes, beginning with the palm savannahs in Bolivia in the foothills of the Andes; changing to the dry forest of Chaco which is the largest of these regions: and then to the great marshland of the Mato Grosso; and ending in the mesophytic forests which extend eastwards as far as the coastal rain forests.

Biodiversity of American flora

Plant biodiversity in the American tropics comprises two aspects which have not yet been studied in full. The first is the diversity of plant types caused by adaptation to a complex, unstable environment where there is much competition. Species have responded to these conditions by forming characteristics that entail complete changes in the organism (epiphytism, parasitism), or by adapting certain organs, as is the case of lianas or floating plants which adjust to more restricted requirements.

The second aspect, the wealth of families, genera and species, has been studied more, but the inventories made in the New World only cover countries or very small regions, which does not give an idea of the complete situation. It is well known that the richness of flora increases from the lower latitudes towards the equator and that, like the diversity of biological forms, it reaches its highest level in the humid tropics. The number of higher plants in the tropical New World is estimated to be between 60 000 and 70 000. A very high percentage of these include the Compositae, Orchidaceae, Rubiaceae, Cyperaceac, Araceae and Melastomataceae families, which have few components that provide materials for consumption or industry. It is to be hoped, therefore, that there is no correlation between a region's wealth of flora and the number of domesticated plants in it. Thus, in Costa Rica, only one of about 1 0000 higher plant species has been domesticated. In Ecuador, whose wealth of flora is unparalleled on the continent, it is surmised that only four species have been domesticated. On the other hand, the areas with the greatest richness of flora are those which offer the best prospects for future use, especially in industry.

Cultural relationships

It is obvious that neither the wealth of flora nor the diversity of the major plant formations are primary factors in domestication. On the contrary, both conditions may be negative: the first because, in a wide range of products, it is easy to find substitutes if one should become scarce; the second because the abundance of a product in a natural formation makes its domestication unnecessary.

Although there is uncertainty as to which species were cultivated in America prior to 1492, their number is estimated to be between 250 and 300. The great majority of them are still at an incipient stage of cultivation and they cannot be categorized as domesticated, if we are to follow the norms laid down by specialists who restrict this concept to those species that have undergone genetic improvement. However, if we accept that the relationship between human beings and cultivated plants has a broader sense, the development and application of cultivation practices and the invention of utilization techniques as factors in the domestication process of a species may be considered to be as important as or more important than genetic improvement.

In tropical America, the paucity of archaeological or historical evidence makes it very difficult to determine the factors which led to domestication and to establish whether this occurred in a limited period or was a long process, whether it occurred in one place or several, and whether it happened just once or recurred in different eras.

Domestication in the New World may have been due to the same causes that were believed to have operated in the Old World: scarcity of harvesting, fishing and hunting resources; population pressure; and environmental changes or cultural transformations. Secondary causes may also be mentioned, such as the convenience of having a resource close at hand rather than having to look for it in the forest, especially if it required only simple handling and propagation methods.

Once the initial stage of domestication had passed, the expansion of crops exposed species to new forces of selection and increased their diversity. Expansion was able to take place by diffusion or migration, as will be described later.

Distribution of genetic diversity

The genetic resources of cultivated plants are not uniformly distributed. A great number of species and varieties are concentrated in tropical and subtropical regions, while they are scarce or completely absent in very extensive areas of temperate zones. Vavilov was the first to report this unevenness in geographical distribution and, although his explanation is still not accepted, his definition of areas with a wealth of genetic resources continues to be valid and useful. There is a close relationship between the eight centres which Vavilov identified as areas of high diversity and the state of agriculture during the discovery. When the Europeans arrived, there were two regions of advanced agriculture in America: Mesoamerica, with centres in Mexico, Guatemala and the Andes, with a similar area in southern Peru. Not only was agriculture more advanced in those two regions, but so were industry, trade, communications and urban development. In these areas, agricultural progress was characterized by a high number of cultivated plants, a small number of domestic animals, the development of irrigation and soil conservation, implements for tillage (for which no draught animals were available) and a food preservation technology which was much more advanced than that which existed in Europe.

Mesoamerica and the Andes had no direct cultural communication. They were separated by an intermediate region, comprising a large part of Central America and northeastern South America, which was far less culturally developed. Several plants were cultivated in both regions, an underlying factor common to the whole continent.

The most widespread cultivated plant which was grown was maize, from the mouth of the Saint Lawrence river in Canada (lat. 52°N) to central Chile (lat. 35°S) and from sea level to 3 900 m altitude. The beans Phaseolus vulgaris and P. lunatus covered a similar area; the latter was cultivated up to the coasts of Brazil. Tobacco and avocados were also common. It is interesting to note that different species of the genera Amaranthus, Capsicum, Cucurbita, Gossypium, Physalis and Pachyrhizus were domesticated in Mesoamerica and the Andes.

Botany, archaeology and history have enabled us to confirm that both regions had a significant number of cultivated native species in their central areas, some of which are still found only in those areas. These nuclear areas were surrounded by others which had different environmental conditions and domesticated plants of local origin. From this, a distribution pattern emerges whereby the number of cultivated species and varieties decreases from the nuclear area of a region to its periphery.

To the local component of germplasm in the nuclear and adjacent areas must be added the foreign contribution from introductions made in different eras and from various origins. Before the discovery, cassava, sweet potato, groundnut and annato (arnatto) were already cultivated, possibly originating from other areas of the continent, while calabash or bottle gourd (Lagenaria siceraria), probably of African origin, was used from the United States down to Argentina for its fruits, which were used as vessels. In the first stages of agriculture, this process of diffusion began through exchanges, theft, war and conquests among the primitive populations. The success of introductions depended on the adaptability of species to new environments and on their acceptance by consumers. The case of the banana is exemplary; it was introduced at the start of the discovery period and its expansion in tropical America preceded the movement of the conquistadores by ten years. The spread of cultivars resulting from the voluntary or forced migration of human communities may have had less effect, but there is historical evidence to confirm its occurrence. The concentration of genetic resources in Mesoamerica and the Andes, which at the time of the conquest were two very extensive empires, can be explained by the accumulation of germplasm throughout a long history of domesticating local crops and adaptating foreign crops on the basis of a few cultivated species common to both regions.

These two regions of intense concentration of germplasm do, however, occupy a very small area. Beyond the tropics, to the north of Mesoamerica, there were a few isolated cases of domestication: sunflower, Jerusalem artichoke (Helianthus tuberosus), Iva annua, Proboscidea parviflora, Chenopodium sp., of which only the first two are known in other regions.

The intermediate zone extends into South America, spanning the basins of the Orinoco, Amazon and Paraguay Paraná. This immense region covers the continent's richest areas in terms of species and environmental diversity. It is here that species of global importance were domesticated: cassava, sweet potato, pineapple, groundnut and many crops in the incipient stages of cultivation. In spite of its enormous size, there are no nuclei with a high concentration of germplasm, and only in the Upper Amazon is there an undefined area of native plants that are still in the incipient stage of cultivation. There is archaeological evidence of extensive ancient human occupation in the intermediate region comparable in age to the occupations of Mesoamerica and the Andes, but no human group attained a culture comparable with that of these civilizations. In order to explain the absence of an advanced agriculture it may be adduced that: in some regions, e.g. the Amazon region, the natural conditions were such that existing plant and animal food resources were sufficient to supply a community with a balanced diet throughout the year; and the low fertility of the soil, the formation of the major rivers and prolonged droughts did not favour the development of a firmly settled civilization.

Finally, in the extreme south of the continent, already outside the tropics, there were isolated cases of domestication. In Chile, the cereals Bromus mango and Elymus sp. were cultivated until European cereals supplanted them; an oleaginous plant, Madia saliva, was cultivated until the eighteenth century. In Argentina and Brazil, cultivation began of Cucurbita maxima, a vegetable of international importance.

The Antilles were not an important source of cultivated plants. Only the mammee (Mammea americana), which appears to be of West Indian origin, grows wild in the Greater Antilles where it is known by the name "taino". It is doubtful whether arrowroot (Marunta arundinacea) was domesticated in the Lesser Antilles as has been suggested; rather, it may have been introduced from South America. The Antilles especially Hispaniola, were where Europeans first became acquainted with and adopted indigenous names for a good number of American cultivated plants. which then spread throughout the continent.

Changes since 1500

Several processes radically affected the state of genetic resources after the discovery and their action varied according to the crop and region while its intensity varied with the era. It is difficult to assess the impact of these processes and to form an accurate idea of the germplasm lost. Historical evidence shows that some species disappeared from cultivation, but it is not possible to evaluate the losses of cultivars.

Two new processes were observed immediately after the discovery. The first was the introduction of livestock, leading to extensive farming which still exists within colonial settings in some regions of Latin America. Extensive livestock farming first occupied the cultivated land that the Indians abandoned either because of the conquest or because of depopulation resulting from disease. To sustain livestock farming on the highlands. Eurasian forage species were introduced while, for the lowlands, African grasses were introduced, some of which turned into obnoxious weeds and practically eliminated the native grasses.

The second process was the introduction of export crops - sugar cane, indigo. coffee, banana, oil-palm which were established by cutting down primary forests or using agricultural land intended for subsistence crops. The monocultures were transformed by dense sowing and agronomic practices into veritable deserts in terms of their scarce biodiversity.

The processes of urbanization, if this is the name that may be given to the building of dwellings and transport routes, seriously affected the preservation of genetic resources since, in Latin America, the expansion of cities mainly took place around the old population centres which were surrounded by intensely and diversely farmed fields. Urbanization brought about changes in diet which reduced the consumption of traditional foods.

The effects that the change in varieties had on the survival of genetic resources were less important in tropical America. First, because of the lack of genetic improvement or selection programmes in the great majority of crops and, second, because of farmers' resistance to adopting new varieties, especially for food crops. (Farmers usually doubt the properties of improved varieties and consequently prefer traditional varieties, when they are not fully informed of the methods for handling them.)

Another factor that favoured genetic diversity was the sowing of mixtures of cultivars in order to ensure that at least one of them was harvested. The aesthetic aspect of this practice is reflected in the diversity of shapes, colours and sizes of fruit, tubers and other products. In some cases, eating habits and the nutritional and culinary superiority of the primitive cultivars contributed to their survival.

The challenge of conservation

In what way can a region comprising 30 independent countries and undergoing serious problems in all areas of development maintain the germplasm of approximately 350 cultivated species? Much is being done at the national and regional level, especially for priority crops that are arousing world interest. However, the food production capacity of most crops has not yet been studied.

Preservation of this genetic stock is a complex task which requires the shared responsibility of national institutions and regional and international bodies as well as political decisions guaranteeing the permanent conservation of germplasm. As the great majority of tropical crops have recalcitrant seeds or are propagated vegetatively, they will have to be conserved in live collections. New techniques will have to be developed for the establishment and handling of collections as well as their duplication in different locations.

Many of the crops of the American tropics are exotic and, therefore, more germplasm and an international policy of introduction and exchange will also be required. This genetic wealth should then ensure rich and varied sources of food and industrial products for the most diverse uses in the future.

Preservation of the native germplasm of tropical America will also depend on the establishment of gene banks and protected areas and will be based on the development of markets and technology for the handling and preparation of agricultural products for consumption. To widen the market for such products, it will be necessary to diversify their uses and to launch information campaigns based on traditional knowledge of their nutritional value and organoleptic qualities. With this aim, ethnobotanical research will need to be stepped up and a systematic study of agronomic management and technologies of product preparation undertaken. Recent experience has shown that, with a little time and effort, it is possible to convert certain neglected crops into supermarket goods. Notable failures have also been seen as a result of insufficient planning and experience. However, in some cases traditional approaches to establishing live collections and seed banks have been successfully combined with research on agronomic management and marketing.

Table 1 lists plants originating in the New World. This catalogue would be incomplete if we did not mention ornamental and medicinal plants.

Ornamental plants

There is historical evidence of the prehistoric cultivation of ornamental genera such as Dahlia, Tagetes and others in Mexico, but it is in the last two centuries that a great number of American species have been introduced for cultivation, first in Europe then in the United States, many of which have been genetically altered. The families that have provided a great number of species include the Araceae. Bromeliaceae, Cactaceae and Palmae. Other families of commercial importance are the Amaryllidaceae, (Hymenocallis, Hippeastrum, Zephyranthes), the Compositae (Cosmos, Tagetes, Zinnia) and the Solanaceae (Browallia, Brugmansia, Nierembergia, Petunia, Salpiglossis). Numerous genera in very different families contain species that are extensively cultivated; for instance the genera Canna, Cleome, Euphorbia, Fuchsia, Ipomoea, Salvia and Verbena. Of special importance are ornamental plants originating from North America, including genera such as Calochortus, Gaillardia, Lewisia, the cultivation of which began in the colonial era and rapidly expanded to other temperate regions.

Contents - Previous - Next